Alkaloids are a group of naturally occurring chemical compounds that mostly contain basic nitrogen atoms.[1] This group also includes some related compounds with neutral[2] and even weakly acidic properties.[3] While the term originates with naturally occurring compounds, some synthetic compounds of similar structure are also termed alkaloids.[4]

Alkaloids are produced by a large variety of organisms including bacteria, fungi, plants, and animals. They can be purified from crude extracts of these by acid-base extraction. They have a wide range of pharmacological properties including antimalarial, antiasthma (e.g. ephedrine), anticancer[5] cholinomimetic (e.g. galantamine),[6] vasodilatory, antiarrhythmic, analgesic (e.g. morphine),[7] antibacterial,[8] and anti-diabetic activities.[9] Although alkaloids act on a diversity of metabolic systems in humans and other animals, they almost uniformly evoke a bitter taste.[10]

Contents

Distribution in nature

Alkaloids are generated by various living organisms, especially by "higher plants" – about 10 to 25% of those contain alkaloids.[12][13] Therefore, in the past the term "alkaloid" was associated with plants.[14]

The alkaloids content in plants is usually within a few percent and is inhomogeneous over the plant tissues. Depending on the type of plants, the maximum concentration is observed in the leaves, fruits or seeds, or root or bark (cinchona).[15] Furthermore, different tissues of the same plants may contain different alkaloids.[16]

Beside plants, alkaloids are found in certain types of psychoactive fungi, such as psilocybin in the fungus of the genus Psilocybe, and in animals, such as bufotenin in the skin of some toads.[17] Many marine organisms also contain alkaloids.[18] Some amines, such as the monoaminesadrenaline and serotonin, which play an important role in higher animals, are similar to alkaloids in their structure and biosynthesis and are sometimes called alkaloids.[19]

History

Alkaloid-containing plants have been used by humans since ancient times for therapeutic, spiritual and recreational purposes. For example, medicinal plants have been known in the Mesopotamia at least around 2000 BC.[20] The Odyssey of Homer referred to a gift given to Helen by the Egyptian queen, a drug bringing oblivion (speculated to be an opium-containing substance).[21] A Chinese book on houseplants written in 1st–3rd centuries BC mentioned a medical use of ephedra and opium poppies.[22] Also, coca leaves have been used by South American natives since ancient times.[23]

Studies of alkaloids began in the 19th century. In 1804, a German chemist isolated from opium a "soporific principle", which he called "morphium" in honor of Morpheus, the Greek god of dreams; in German and some other Central-European languages, this is still the name of the substance. The term "morphine", used in English and French, was given by a French physicist.

A significant contribution to the chemistry of alkaloids in the early years of its development was made by French researchers, who discovered quinine (1820) and strychnine (1818). Several other alkaloids were discovered around that time, including xanthine (1817), atropine (1819), caffeine (1820), coniine (1827), nicotine (1828), colchicine (1833), sparteine (1851), and cocaine (1860).[24]

The first complete synthesis of an alkaloid was achieved in 1886 by the German chemist Albert Ladenburg. The development of the chemistry of alkaloids was accelerated by the emergence of spectroscopic and chromatographic methods in the 20th century, so that by 2008 more than 12,000 alkaloids had been identified.[25]